1. Field of the Invention
The present invention relates to ink jet printers, and, more particularly, to a method of printing with an ink jet printer having two inks of the same hue but of different saturation.
2. Description of the Related Art
Computer applications that require printing generally utilize a laser or ink jet printer which is connected to the computer. These computer driven electronic printing devices print in a binary manner wherein the output medium or image is divided into an array of picture elements, pixels, or pels. A printing device can print a dot onto a print medium at each pixel location or leave the pixel location blank. In the case of mono-color printers, all of the dots are printed with a single color, with the resulting output consisting of a 2-dimensional array of colored and blank pixels.
However, pictorial images such as those produced by computerized imaging, are continuous in tonality between either black (a printed dot for pixel) or white (blank pixel). If such a continuous tonality image is divided into pixels, each pixel could be assigned a grayscale color whose tonal value falls within a range of tonal values. In order to reproduce such continuous tonality images by electronic printing devices, it is necessary to assign each pixel a continuous tonality value.
During the printing process, the image to be printed is divided into a series of pixels and the value of the image in each pixel is quantized to produce a multi-bit digital word which represents the tonal value of the pixel. The image is thus converted into a bitstream of words which is provided to the electronic printing device. In order to convert the image data pixel value format into a format suitable for use by the electronic printing device, a half-toning process is performed on the word bitstream. The half-toning process may take many forms having many names. In one form, a comparison is made between each image data pixel value or word in the bitstream with a threshold value or level, and a binary output pixel value is generated depending on the relative values.
In one example, a continuous tonality processor for an image might generate a bitstream of numerical values representing the detected light intensities of the pixels. Commonly, the numerical value would be in a range of 0-255, corresponding to a 256 level gray scale or an eight-bit word. Then, when the bitstream is to be produced on the electronic printing device, the words of the image data are compared to either a single threshold value or an array of threshold values to produce the required binary output pixel stream, according to the half-toning process.
This half-toning process, however, can produce a loss or distortion of visual information between the original image and the printed image. Techniques have been developed to reduce this distortion. One such technique is error diffusion. Another technique, known as dithering, is an alternative to error diffusion. Dithering has been developed to reduce the number of computations necessary to process an image. A dither or threshold array is an array of predetermined (and generally different) threshold values with the same spacing as the image pixels. In this manner, the dither array is conceptually overlaid onto the image pixel array. If the dither array is smaller than the image array, then the dither array is repeated side-by-side or tiled over the image array to produce a repetitive pattern. Each pixel thus has two values associated with it, the actual image pixel tonal value and the dither or threshold value of the overlaid dither array cell to which it is compared.
One type of electronic printing device that may utilize the half-toning process is the ink jet printer. Ink jet printers typically include a printhead assembly which is carried by a carriage assembly which is scanned or moved across a print medium, such as paper, in a direction transverse to the feed direction of the paper. The printhead is generally carried on an ink jet cartridge assembly that has an ink reservoir. For a mono-color printhead used to jet a single color ink, e.g., black ink, onto the print medium, the printhead is moved across the print medium in one transverse direction, advanced a distance corresponding to the height of the printhead, and moved in a return direction back across the print medium in an opposite direction. During printing, ink is jetted from the ink emitting orifices in the printhead onto the paper as the printhead moves in transverse directions across the print medium. More particularly, as the print head assembly is scanned across the paper, ink is selectively jetted from any or all of the available nozzles in the print head assembly.
The image to be printed onto the print medium may be stored as data. The image can be represented as a collection of picture elements, otherwise referred to as pixels or pels that can be further represented by ink dots. It is also known to divide a plurality of pixels making up an image area into a plurality of rows of pixel locations or cells and a plurality of columns of pixel locations or cells.
By sequentially scanning the printhead across the print medium and advancing the print medium between the lateral scans a distance corresponding to the height of the printhead, ink may be selectively jetted onto the print medium at any pixel location within the image area. Various software algorithms are used in ink jet printing that generally control the timing, sequence and/or placement of the ink dots from the ink jet print head on the paper. Examples of such software algorithms include shingling and dithering with dithering accomplished by application of dither or threshold arrays.
In utilizing a dither array for gray scale printing in ink jet printers, ink dots are placed at selected locations within an image area by the print head. Typically, a single ink is used in association with a dither array during gray scale printing. Image data includes data for a single pixel location with an assigned gray scale value. If the gray scale value is greater than the threshold value within the dither array at the same pixel location, then the ink dot of the single color ink is placed at that pixel location. Dither or threshold arrays for gray scale printing provide the advantage of being computationally fast.
It is becoming more common for an ink jet printer to include multiple printheads, each printhead having ink of the same hue but of a different saturation. However, in order to use dithering with the same hue, different saturation ink, each print head/ink saturation must have a dither array associated therewith. The use of two dither arrays or, in general, using a dither array for each print head, slows down computation time. Dither arrays have heretofore been used with single density ink.
What is thus needed is a faster method for gray scale printing in an ink jet printer having multiple print heads associated with same hue, but different saturation ink.